(1) Field of the Invention
The present invention relates to a doze detector for detecting a doze on the basis of a degree of opening and closing of human's eyelids by the use of reflection sensors each including a pair of a light emitting element and a light receiving element. Further, the present invention relates to an alarm generating system using the doze detector.
(2) Description of Prior Art
Heretofore, a doze detector using a reflection-type photoelectric switch, as a sensor, including a pair of a light emitting element and a light receiving element has been proposed. Such a doze detector has been arranged in the following manner. The sensor is mounted on a frame of a pair of spectacles so that light is projected onto a human eye from the light emitting element of the sensor, and the reflected light from the human eye is received by the light receiving element of the sensor. At this time, the reflection point varies, that is the distance from the light emitting element varies, as a result of which the quantity of light received by the light receiving element at the opening time of the eyelids is different from that at the closing time of the same. The opening/closing of the eyelids is detected by discrimination of the level of the quantity of light reception, so that a dose is detected on the basis of the time of closure of the eyelids, and an alarm is generated by a buzzer or the like to inform the occurrence of a doze.
With such a prior art doze detector as described above, the quantity of reflected light changes as the eyeball moves even in the case where the eye is opened. There is a possibility that the light quantity may be increased to that at the closing time of the eyelids resulting in an occurrence of maldetection in detecting the condition of eyes due to the motion of the eyeball, that is, the movement of an iris.
For example, assuming that light from a light emitting element la of the detector is projected onto an area of the eye indicated by a broken line in FIG. 1 and the light reflected from the area of the eye is received by a light receiving element lb, the quantity of reflected light when the eyelids are closed is larger than that when the iris exists in actual in the area of the eye. Accordingly, the voltage level in an output of the light receiving element lb2 becomes higher as shown in FIG. 3(a) when the eyelids are closed. Thus, in the prior art doze detector, the closing of the eyelids is detected in the condition that the level voltage exceeds a predetermined threshold level T.sub.H and the OccurrenCe of a doze is recognized by detecting the case where the eyelids are closed continuously over a predetermined period of time. Even in the case where the eyelids are opened, if the iris moves right as shown in FIG. 2, the quantity of the reflected light received by the light receiving element 1b is not so different from that obtained in the case where the eyelids are closed. This is because, when the iris of the eye is in the right, the light reception area shown by the broken line is occupied by the white of the eye. 0n the contrary, the light reception level from the white of the eye is often higher than that obtained in the case where the eyelids are closed as shown in FIG. 3(b). Consequently, in the case where the iris of the eye is in the right, the quantity of reflected light becomes high as if the eyelids were closed. In this condition, therefore, the signal is processed in the same manner as in the condition where the eyelids are closed in spite of the fact that the eyelids are not closed, and the detector is likely to detect a doze erroneously when this condition is continued over a predetermined period of time.
In the case where car drivers use a pair of spectacles provided with such a doze detectors, the aforementioned maldetection often arises because the car drivers must move his eyeballs left and right in accordance with circumstances or in other words the iris and the whites of their eyes must move left and right. Accordingly, in this case, there may occur a problem in safe driving.
FIG. 4(a) is a waveform diagram showing the variations in the output signal of the light receiving element lb due to the blinks and the closing of the eyes. As mentioned above and as shown in FIG. 4(a), the output signal of the light receiving element 1b of the conventional doze detector exceeds the threshold level V.sub.th for a relatively long period of time when the human falls into a doze b and thus the eye is closed continuously. 0n the other hand, upon an occurrence of blinks a and a as shown in FIG. 4(a), the output signal also exceeds the threshold level V.sub.th for a short time. Therefore, by detecting such a condition that the output signal exceeds the threshold V.sub.th continuously for the relatively long period of time, for half a second for instance, the doze detector recognizes the doze and then generates an alarm such as a buzzer or the like.
In the aforementioned conventional doze detector, the alarm is not generated until the user has fallen into a doze. As a result, a time lag arises between the point in time of initiation of a doze and the point in time of actuation of the alarm. Therefore, the conventional doze detector has a disadvantage in that accidents may happen during the time lag.
An operation of the conventional doze detector will .be- described in detail with reference to FIG. 4(b) which is a flow chart for the operation thereof. When an electric power switch of the detector is turned on, the system of the doze detector is initialized [Step (hereinafter abbreviated as "ST") 41]. The light from the light emitting element la is projected onto the eyeball, and the reflected light is received by the light receiving element lb to thereby detect a blinking condition (ST 42).
As described above, there is a difference in light intensity between the light reflected from the eyeball and the light reflected from the eyelids. This is because the distance between the eyelid and the reflection sensor is shorter than that between the eyeball and the reflection sensor, and the light reflected from the skin-color eyelid is more intensive than that reflected from the black cornea of the eye. Accordingly, a reference voltage is set in advance to be successively compared with the intensity of light (the quantity of received light) fed from the reflection sensor. For example, as shown in FIG. 4(a), in the case where the driver is in an awakening state and he does not blink, the emitted light is reflected from a portion corresponding to his eyeball including the iris so that the quantity of received light is smaller than the reference voltage that is a threshold level for a comparator 6. In this case, the eye closing time due to a blink is very short. Accordingly, upon blinking, it is detected in the ST 43 that the eye-closing time of a blink is shorter than half a second so that the operation returns to the ST 42. On the contrary, in the case where the driver falls into a doze, the eye-closing time exceeds half a second. Accordingly, in this case, the output signal having an output level higher than the threshold level is continuously produced over half a second. As the result, it is detected in the ST43 that the eye-closing time exceeds half a second, and then the operation is allowed to advance to the ST 44 where the alarm is generated for three seconds to inform the car driver of a doze state.
In the aforementioned conventional doze detector, a doze state is recognized only when an output signal having a voltage level exceeding the reference voltage is continuously produced for a predetermined period of time, over half a second for instance.
However, the state in which the driver blinks with the eye-closing time not shorter than half a second means a sleeping state in which the driver falls into a perfect doze. Accordingly, with such a conventional detector, an alarm is not generated during a drowsiness state before the driver falls into a perfect doze. Hence, it is insufficient to attain an object of the doze detector for keeping the car driver in the awakening state.